csdp: move gfortran to nativeBuildInputs

pkgs/applications/science/math/csdp/default.nix

···
     sha256 = "1f9ql6cjy2gwiyc51ylfan24v1ca9sjajxkbhszlds1lqmma8n05";
   };
 
-  buildInputs = [ blas gfortran.cc.lib lapack ];
+  nativeBuildInputs = [ gfortran ];
+
+  buildInputs = [ blas lapack ];
 
   postPatch = ''
     substituteInPlace Makefile --replace /usr/local/bin $out/bin